Start here:
https://www.unrealengine.com/en-US/onlinelearning-courses
YT can be OK but it’s not guided - you must know what you want to learn first and quality varies greatly.
While the official documentations may seem somewhat lacklustre here and there, the elementary bits are covered; this could be a starting point if you prefer reading to watching:
https://docs.unrealengine.com/en-US/GettingStarted/index.html
how long will take me to be able to
start game development and will that
help me to understand c++
Technically, you can start now - learn by making very simple games or even their isolated features. And the overall understanding of blueprints will help with learning C++ later on. It works both ways.